Krebs said hackers were possibly in Home Depot's computer systems from May until now. If that's true, this might be even larger than the three-week long Target breach that affected 40 million debit and credit cards late last year, he noted.

In a statement, Home Depot spokeswoman Paula Drake said: "Protecting our customers' information is something we take extremely seriously, and we are aggressively gathering facts at this point while working to protect customers."

The company promised to alert customers as soon as it can ascertain a data breach has occurred.

For perspective, consider that Target(TGT) is still reeling from its brush with hackers. The company's latest figures estimate the damage so far at $148 million -- and that number continues to rise. The value of its stock has fallen nearly 5% this year, and the company's CEO resigned.